Redundant length in array instances

The `Binary` instances for `Array i e` and `UArray i e` serialise the length of the array. This is redundant, since the length can be computed from the bounds (which are also serialised).

Getting rid of the length would save 8 bytes. The question is if it's worth it to potentially break code for this (small?) gain.

Start by locating the Binary instances for Array i e and UArray i e and reviewing how their bounds and lengths are serialized. Determine whether removing the length is compatible with existing serialized data and whether the 8-byte saving justifies the break; the issue is done only once that compatibility question has a clear decision and agreed scope.

Written by the indexing model from the issue text.